Rolling Resistance - Tyre Type and Pressure

[Tony Griffiths]

Mrs. Google tells me:
 
The relationship between frictional force and the area of contact is a nuanced topic in physics. Here's a breakdown of the key points:

1. Frictional Force Basics: The frictional force (Ff) between two surfaces is generally given by the equation:
   Ff=μFn
   
2. where μ is the coefficient of friction (which depends on the materials in contact) and Fn is the normal force (the perpendicular force pressing the two surfaces together).
   
3. Area of Contact: Intuitively, one might think that a larger area of contact would lead to a larger frictional force. However, in many cases, the frictional force is independent of the contact area when the normal force is constant. This is because while a larger area may distribute the force over a larger surface, it also reduces the pressure (force per unit area) on any given point of contact.
   
4. Pressure Consideration: The frictional force is more strongly influenced by the pressure between the surfaces. For a constant normal force, increasing the area decreases the pressure, which can lead to a decrease in the microscopic interactions that contribute to friction.
   
5. Exceptions: There are some scenarios, particularly in materials with very high surface roughness or in cases involving adhesives, where the area of contact can play a more significant role in determining the frictional force.
   
6. Conclusion: In general, for most practical purposes and under normal conditions, the frictional force does not depend on the contact area for rigid bodies in dry friction scenarios. The key factors are the nature of the surfaces and the normal force applied.
   

Indeed -  the chemical (rubber compound) and mechanical actions between the tread and road surface are important - and in practice, wider tyres do, in general, give greater grip under 'normal' conditions, though, of course, on snow and standing water, wide tyres are rather less happy.  I once had the front tyres on a UNO Turbo replaced, drove away from the garage on dry roads and was appalled at the instant and massive understeer. Upon checking, the garage had mistakenly fitted tyres far narrower than those specified. Check YouTube videos of two identical BMWs fitted with summer and winter tyres of the same size accelerating away on a wet track. It's all very interesting....